This invention relates to the improvement of heat transfer in heat exchangers and in particular to heat exchangers in which process fluids are circulating which have a tendency to coat or "foul" the inside tube surface. Such fouling coatings produce a thermal resistance which inhibits heat transfer and lowers the heat transfer coefficient of the tubing.
When fouling conditions during operation are anticipated, it is common practice to build heat transfer units much larger than if no fouling was expected. For example, the amount of tubing required for a given job could be selected during the design stage in accordance with the decreased heat transfer efficiency to be expected after a pre-determined amount of fouling had taken place. A system can also be designed so that different sections can be sequentially taken out of service to permit cleaning to take place in one section while other sections remain in operation. Either of these measures takes an economic toll.
It is common to clean fouled surfaces by circulating a solvent or other type of cleaning fluid through the tubing as exemplified in Tyden U.S. Pat. No. 2,490,759 and Matthiesen U.S. Pat. No. 3,647,687. Obviously, such a cleaning process removes the heat transfer unit from operation and would thus interupt the process operation in which the unit was used. U.S. Pat. No. 3,211,217 to McKee et al shows a multipass heat transfer unit designed to be used with cooling fluid such as river water which commonly contains a large amount of debris which tends to collect in the inlet ends of the plain tubes so as to obstruct them. A valve in the unit reverses the flow of fluid through the tubes and dislodges the debris which then passes through the drain. Since the tubes are disclosed as being plain, the reversal of flow direction could not be expected to be effective in abating relatively uniform fouling coatings such as microcrystalline scale or sludge. Wolfe, Jr. U.S. Pat. No. 3,450,193 teaches that corrugated tubing should be used in only a single flow direction and that the inlet end should be of larger diameter than the outlet.